Breakdown protection for field emission electron gun

ABSTRACT

This invention relates to a field emission type electron gun capable of protecting the emitter tip from damage when electrical breakdown occurs in the gun chamber. The preferred embodiments incorporate circuitry for decreasing the impedance between the emitter and its associated electrode when electrical breakdown occurs.

United States Patent 1191 Aihara et al.

[ BREAKDOWN PROTECTION FOR FIELD EMISSION ELECTRON GUN [75] Inventors: Ryuzo Aihara, Machida; Susumu Ota, Akishima; Nobuyuki Kobayashi, Kodaira, all of Japan [73] Assignee: Nihon Denshi Kabushiki Kaisha, Tokyo, Japan Notice: The portion of the term of this patent subsequent to May 7, 1991 has been disclaimed.

[22] Filed: Oct. 26, 1973 [21] Appl. No.: 409,918

Related US. Application Data [63] Continuation-impart of Ser. No. 245,232, April 18,

1972, Pat. No. 3,810,025.

[30] Foreign Application Priority Data ]*Nov. 18, 1975 [56] References Cited UNITED STATES PATENTS 2,083,205 6/1937 Schlesinger 328/10 2,129,088 9/1938 George 328/10 2,916,668 12/1959 Dyke et a1. 313/336 X 3,084,284 4/1963 Schultz et a1. 328/9 3,267,321 8/1966 Gessford, Sr. 323/10 X 3,553,522 1/1971 Feaster 328/10 X 3,678,333 7/1972 Coates et a1. 250/311 X 3,691,377 9/1972 Matsui et al. 315/307 3,760,180 9/1973 Weber 250/311 X 3,810,025 5/1974 Aihara et a1. 328/10 Primary Examiner-William D. Larkins Attorney, Agent, or Firm-Webb, Burden, Robinson & 'Webb [57] ABSTRACT This invention relates to a field emission type electron gun capable of protecting the emitter tip from damage when electrical breakdown occurs in the gun chamber. The preferred embodiments incorporate circuitry for decreasing the impedance between the emitter and its associated electrode when electrical breakdown occurs.

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BREAKDOWN PROTECTION FOR FIELD EMISSION ELECTRON GUN RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 245,232 filed Apr. 18, 1972 now US. Pat. No. 3,810,025 which claimed priority under 35 U.S.C. Section 1 19 to Japanese applications Ser. Nos. 46-30876, 46-31405, and 46-69070 filed Apr. 20, May 1 l, and Sept. 7, 1971 respectively.

This invention relates to a field emission type electron gun capable of protecting the emitter tip from damage when electrical breakdown occurs in the gun chamber. The advantage of a field emission type electron gun in electron microscopes and the like as compared with the ordinary thermionic emission type electron gun, is that it is possible to obtain a high current electron beam forming a microspot. Unfortunately, however, in the case of the field emission type gun, the emitter is very easily damaged due to electrical breakdown caused by a deterioration in the gun chamber vacuum or other phenomena, resulting in the generation of an unusually strong electric field in the vicinity of the emitter tip. As a result the tip is overheated due to large current during vacuum discharge. This inevitably results in a change in shape in the emitter tip which then becomes useless.

It is a principal object of this invention to protect the emitter against the effects of electrical breakdown.

Briefly, according to this invention, a field emission electron gun comprises a circuit for preventing an unusually high potential difference between the emitter and an electrode spaced near theemitter resulting from an electrical breakdown between that electrode and the grounded anode. The preferred means of preventing the damaging high potential difference is to provide an electronic switching circuit between the emitter and electrode which will rapidly reduce impedance and the potential difference therebetween at the time of breakdown and preferably a circuit to increase impedance between the high voltage source and the emitter at the time of breakdown.

Further features of this invention will become apparent by reading the following detailed description in conjunction with the accompanying drawings in which:

FIGS. 1(a) and (b) are schematic diagrams of a conventional field emission type electron gun.

FIG. 2 is a diagrammatic circuit of one embodiment according to the invention.

FIG. 3 is a diagrammatic circuit of an embodiment according to this invention having an additional electrode.

FIGS. 4(a) and 4(1)) are graphs showing the electrical characteristics of electronic switches according to this invention.

FIG. 5 is a diagrammaticcircuit of another embodiment according to this invention having an additional electrode.

FIGS. 6 and 7 are a diagrammatic circuit and physical equivalent of an embodiment according to this invention designed to increase the impedence in series with the emitter during discharge.

Referring to FIG. 1(a), in a typical field emission electron gun, a gun chamber 1 contains a filament 2 heated by an AC. current source 3 through an insulation transformer 4, an emitter 5 attached to the fila- 2 ment 2, first electrode 6 for producing a strong electric field (for example, about 10 volt/cm) in the vicinity of the emitter tip and an anode 7 maintained at ground potential. A voltage source 8 supplies a voltage so as to create a constant or pulsed potential difference between the emitter 5 and the first electrode 6, in order to draw electrons from said emitter tip. Another voltage source 9 maintains the emitter at a high negative DC. potential in order to accelerate the emitted electrons.

In this electron gun, electrical breakdown occurs mostly between the firstelectrode 6 and the anode 7, due to the high potential difference existing there compared with that existing between the first electrode 6 and the emitter 5.

In FIG. 1(1)), Z10 and Z11 represent the impedances of sources 8 and 9 (FIG. 1(a)) respectively, and Z12 the impedance between the twosources.

When electrical breakdown occurs between the first electrode 6 and the anode 7, the potential of the first electrode 6 becomes zero (ground) and discharge current flows through impedances Z10, Z12 and Z11. Accordingly, if the impedance Z10 is not sufficently small as compared with the sum total of impedance Z11 and Z12, the potential difference between the emitter 5 and the first electrode 5 will be large, resulting in damage to the emitter tip.

FIG. 2 illustrates one embodiment of this invention in which the above possibility, viz., damage to the emitter tip, is eliminatedby providing a protection circuit.

Referring to FIG. 2, the protection circuit 13 according to this invention is arranged in parallel with voltage supply source 8, said circuit comprising a thyratron 14, a variable DC. voltage source 15 for adjusting the firing voltage of the thyratron l4, and a coupling condenser 16.

When. surge voltage, resulting from electrical breakdown, is applied to circuit 13; that is to say, across source 8, the thyratron switches to its conducting condition thereby protecting the emitter tip from damage due to said surge voltage.

FIG. 4(a) shows the general shape of the plate voltage-current curves for a thyratron electronic switch useful according to this invention and as used in the embodiment explained with reference to FIG. 2. When the ionization potential is reached, the impedance of the tube drops to near zero.

FIG. 3 illustrates yet another embodiment of the invention in which an additional electrode 18, hereinafter referred to as the second electrode, is provided between the first electrode 6 and the anode 7. The potential applied to the second electrode 18 is determined by a DC voltage source 19 and the voltage source 9. In this case, the output voltage of the voltage source 19 is almost the same as that of source 8 if the output of source 8 is DC, and is almost the same as the pulse height of the output source 8 if the output of source 8 is pulse. Moreover, the output voltage of source 19 is much smaller than the output voltage of source 9. Accordingly, electrical breakdown between the second electrode 18 and the anode 7 almost always occurs before the occurrence of breakdown between the second electrode 18 and the first electrode 6.

The moment the breakdown occurs between the second electrode 18 and the anode 7, the potential of the second electrode 18 changes from negative high potential to ground potential. As a result, the potential difference between the second electrode 18 and the first electrode 6 increases drastically due to the sudden outflow of discharge current through the impedance of DC. voltage source 19. This, in turn, activates protection circuit 13, comprising simple thyristors 20, which switch into the conducting condition thereby decreasing the impedance between the two electrodes to near zero, and in so doing, prevents the reoccurrence of electrical breakdown since the potential fluctuation of said electrodes and the emitter 5 are about the same.

FIG. 4(b) shows the general shape of the voltage-circuit curves of a bidirectional diode thyristor electronic switch as used in the embodiment described with reference to FIG. 3.

An additional advantage of the embodiment shown in FIG. 3 is the fact that the range of the protection circuit firing voltage is much wider than that of the protection circuit described in the embodiment shown in FIG. 2. This is because, in the case of the embodiment shown in FIG. 2, it is necessary to adjust the firing voltage in accordance with the output voltage of voltage source 8 which has to be varied every time the emitter is ex changed. In the case of this embodiment. however, the relevant voltage source is source 19 which is seldom varied. Hence, in the case of this embodiment, there is almost no necessity to adjust the firing voltage of protection circuit 13. For this reason, it is possible to utilize a simple thyristor 20 switching device in place of the more complicated thyratron and its associated circuit.

The embodiment illustrated in FIG. 5 is substantially the same as that illustrated in FIG. 3. In this embodiment. however, an insulation column 21 containing an insulation transformer 4, a high voltage source 22 and a filter circuit consisting of a resistor 23 and capacitors 24 is connected to a gun chamber circuit 25 by high voltage cable 26. Further, the potential at the junction of balancing resistors 27 and 28 is used instead of the potential of the emitter 5, and protection circuit 13 which, in this case, incorporates a surge voltage protection tube 29, is connected between said junction and the second electrode 18.

The surge voltage protection tube should have little or no impedance in the conducting state.

In the embodiment shown in FIG. 5, theoretically speaking, when electrical breakdown occurs between the second electrode 18 and the anode 7, vacuum arc discharge between the emitter and the first electrode 6 does not occur. In practice, however, if the residual impedance in the protection circuit is fairly large, the potential difference between electrode 6 and electrode 18 will be correspondingly large due to the outflow of discharge current.

The peak voltage E of the potential difference is given by the following equation:

where. Z30 represents the impedance between the second electrode 18 and the emitter 5, Z31 represents the impedance of the difference between the impedance Z30 and the total impedance existing in the discharge path of the discharge current due to electrical breakdown, C represents the stray capacity between electrode 6 and the emitter, and C represents the stray capacity between electrode 6 and electrode 18.

The embodiments shown in FIGS. 3 and 5 are designed to decrease impedance Z30 during discharge so LII as to keep E as low as possible, that is to say, minimize the potential difference between electrode 6 and electrode 18.

FIGS. 6 and 7 illustrate an embodiment designed to further reduce E by increasing the impedance Z31 during discharge in addition to decreasing the impedance Z30.

FIG. 6 illustrates an embodiment designed to increase the impedance Z3l inductively. For this purpose, the embodiment incorporates a circuit 32 consisting of coils L11, L12, L4N and capacitors C1, C2, ...CN. Coils L11, LIN, L21, L2N and L31,. L3N forming part of said coils are required to have the same inductance and be unidirectionally wound, whereas coils L41, L4N may have a different or the same inductance and may be wound in the opposite or the same direction to the above coils.

FIG. 7 illustrates the physical equivalent of the circuit 32 shown in FIG. 6 in which conducting wires 46, 47, 48 and 49 leading from the high voltage cable 26 are wound on a core 50. By so doing, it is possible to eliminate capacitors C1, C2, etc., so long as the stray capacity between the conducting wires is adequate.

Accordingly, in the embodiment shown in FIG. 6, discharge current flows through circuit 32 which has a fairly high inductance; whereas, under normal operating conditions, the inductance of circuit 32 is cancelled out by the coils and is, therefore, zero. For example, the pulse signals generated by pulse generator 39 are transmitted to the input winding of the transformer 42 and capacitor CN+I via the transmission line consisting of coils L21, L2N, L31, L3N and capacitors C1, CN without loss. Moreover, resistor 51 connected across the output winding of transformer 42 compensates for the sag in the pulse voltage applied between the emitter 5 and the first electrode 6. The filament heating current is D.C. rectified by rectification circuit 52. Even if A.C. current is used for heating the filament, there is practically no loss in circuit 32, because coils L11, LlN and coils L31, L3N have the same inductance and are unidirectionally wound.

It is possible, of course, to incorporate coils of circuit 32 between the high voltage source 9 and the junction of the emitter and voltage source 8 in the embodiment shown in FIG. 2.

We claim:

1. A field emission type electron gun device comprismg:

i. an emitter for emitting an electron beam,

ii. an anode for accelerating the electron beam,

iii. a high voltage source for supplying negative high potential to said emitter in order to accelerate said electron beam,

iv. an electrode located between said emitter and said anode,

v. a voltage source for generating a potential difference between said electrode and said emitter in order to generate a strong electric field in the vicinity of said emitter tip, and

vi. a protection means for preventing the generation of an unusually high potential difference between said emitter and said electrode, comprising an electronic switch for rapidly shorting the electrode and emitter in the event that the voltage between said emitter and said electrode increases beyond the firing voltage of the switching circuit, and

vii. means for heating the emitter and maintaining the potential thereof comprising a delay line comprising nected to said emitter, the second delay line coil I being connected at one end to said high voltage source (iii) and at the other end through balancing resistors to each side of said emitter and through said protection circuit to said electrode (iv). 

1. A field emission type electron gun device comprising: i. an emitter for emitting an electron beam, ii. an anode for accelerating the electron beam, iii. a high voltage source for supplying negative high potential to said emitter in order to accelerate said electron beam, iv. an electrode located between said emitter and said anode, v. a voltage source for generating a potential difference between said electrode and said emitter in order to generate a strong electric field in the vicinity of said emitter tip, and vi. a protection means for preventing the generation of an unusually high potential difference between said emitter and said electrode, comprising an electronic switch for rapidly shorting the electrode and emitter in the event that the voltage between said emitter and said electrode increases beyond the firing voltage of the switching circuit, and vii. means for heating the emitter and maintaining the potential thereof comprising a delay line comprising two coils and a reactive coil wrapped on a common core, an insulating transformer the secondary of which is connected to a first coil of the delay line and the reactive coil and the primary of which is connected to an A.C. source, the opposite leads to said first delay line coil and said reactive coil being connected to said emitter, the second delay line coil being connected at one end to said high voltage source (iii) and at the other end through balancing resistors to each side of said emitter and through said protection circuit to said electrode (iv). 